I. Field of Invention
This invention relates to batteries. Specifically, the present invention relates to systems and methods for indicating a charged state of a rechargeable battery and for indicating when the battery should be replaced.
II. Description of the Related Art
Batteries are designed to provide approximately constant voltages to a variety of applications ranging from electronic amplifiers and receivers in wireless phones to automobiles. Such applications often require accurate battery gauges or indicators to indicate remaining battery life. Accurate battery life indicators are particularly important in applications such as wireless phones, where rechargeable batteries are common, and knowledge of remaining battery life is desirable.
A battery often includes two metal electrodes immersed in an electrolyte having a depolarizing mix. Batteries are often named in accordance with the type of electrolyte employed such as nickel-cadmium, alkaline-manganese, lead storage, mercury, zinc-carbon, and lithium.
Typically, batteries are designed to approximate ideal voltage sources. However, unlike ideal voltage sources, which have zero internal resistance, batteries have non-zero internal resistance, which is often neglected in the design of electrical systems such as remaining battery life indicators. The internal resistance of a battery varies with battery age but remains relatively constant over short time periods, such as during charging and discharging. As a rechargeable battery ages over several charging cycles, its internal resistance increases as its chemistry, i.e., electrolyte and/or depolarizing mix, degrades. As a battery's chemistry degrades, its ability to hold a charge also degrades.
A battery's performance is often characterized by a discharge curve, which is a curve depicting battery voltage as a function of time given a predetermined discharge rate, i.e., battery current draw. The discharge curve often drops dramatically at a discharge curve knee.
Discharge curves vary with the internal resistance of the battery, the battery discharge rate, and temperature. This is particularly true for zinc-carbon batteries. As the discharge curve changes, so does the capacity and life of the battery. In addition, discharge curves vary in accordance with battery type. Current trends in re-chargeable battery technology are resulting in discharge curves with sharper knees, which may magnify errors associated with conventional battery life indicators.
As a battery ages, its internal resistance generally increases. Hence, battery voltage output, as depicted by the discharge curve, varies not only with temperature and discharge rate but also with battery age. A high discharge rate, a drop in temperature, or an extended storage period reduces battery capacity and life span and causes the discharge curve to drop-off sooner. Most batteries have a limited shelf life, which is the time required for the voltage of an unused battery to drop below a predetermined percentage of its original voltage.
To display remaining battery life, one approach measures the battery voltage and displays a value equivalent to the ratio of the measured voltage to the voltage of a new battery as an indication of remaining battery life. Unfortunately, such gauges are often deceptive and do not account for the knee in the voltage discharge curve or changes of the discharge curve with temperature, discharge rate, battery age, and so on. For example, when the state of the battery nears the knee in the discharge curve, the gauge may indicate that the battery is approximately fully charged, when in fact, it will soon require recharging.
An alternative approach involves accounting for an average shape of the discharge curve of a new battery and mapping voltage values from the average discharge curve into a set of values that varies linearly with time. In this way, when the battery voltage nears the knee in the discharge curve, the battery life indicator is more accurate and is less likely to drop off suddenly as the discharge curve reaches the knee. This approach, however, has significant limitations.
As the battery ages, the discharge curve changes from that of a new battery as the internal resistance of the battery changes. Over time, remaining battery life as indicated by simply measuring the battery voltage and accounting for an average discharge curve of a new battery, quickly becomes inaccurate. This approach also does not account for changes in a battery's discharge curve due to changes in the discharge rate or changes in temperature. Hence, when significant current is drawn from the battery, such as during a phone call via a wireless phone employing the battery, the battery indicator will display erroneous information. For example, many phones display significantly reduced battery life immediately after a call. Shortly thereafter, the battery life indicator rises as though the battery is recharging itself.
Hence, existing systems and methods for indicating remaining battery life typically do not account for changes in a battery's discharge curve as the battery ages and its internal resistance increases. As the battery ages, voltage dropped across the battery's internal resistance increases, changes the discharge curve, and adversely effects the accuracy of the battery indicator or gauge. In addition, as more current is drawn from the battery, more voltage drops across the internal resistance of the battery, which also changes the discharge curve. This introduces errors in battery gauges that are based primarily on battery voltage alone. As a battery ages, predetermined battery voltages used to estimate remaining battery life become less appropriate as they are chosen based on the discharge curve of a new battery.
Hence, a need exists in the art for a system and method for accurately displaying the remaining life of a rechargeable battery. There is a further need for a system that accounts for variations in the internal resistance of a battery with age and variations in a battery's discharge curve with discharge rate.